1. Field of the Invention
This invention relates generally to a material that has giant magnetoresistance properties and a method for making the material. Materials exhibiting giant magnetoresistant effect are being investigated for use in read heads for high-density magnetic recording and other applications requiring small magnetic sensors that are more sensitive than conventional permalloy heads.
2. Description of Related Art
Magnetoresistance is the change in electrical resistance of a material under the influence of a magnetic field (McGraw-Hill Encyclopedia of Science and Technology, 7th ed. (McGraw-Hill: New York, 1992). In 1988, the magnetoresistance of certain preparations of alternating layers of magnetic and nonmagnetic superlatices was found to be very large and were termed "Giant Magnetoresistance". For example, the resistivity of superlattices of iron and chromium, when there is antiparallel coupling or antiferromagnetic coupling between adjoining iron layers 30 .ANG. thick separated by 9 .ANG. chromium layers and no external magnetic field, was lowered by almost a factor of 2 when a magnetic field of 2 Tesla (T) was applied at a temperature of 4.2 K. Typically about 30 bilayers were grown by molecular beam epitaxy (Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices, Baibich, M. N. et al., Phys. Rev. Let. 61(21): 2472, Nov. 21, 1988. Preparation of giant magnetoresistant (GMR) material by use of molecular beam epitaxy methods is too costly and too slow to be useful for commercial applications. The practical utility of GMR in antiferromagnetic coupled multilayer materials was further limited by the large magnetic fields (2-4 T) required to exhibit the effect.
The GMR mechanism appears to be based on spin-dependent scattering of the conduction electrons within the magnetic particles and, more importantly for GMR, at the interfaces between magnetic particle and the nonmagnetic metal matrix. The magnitude of the GMR effect is expressed as .DELTA.R/R.sub.sat .tbd.(R.sub.H-0 -R.sub.H-sat)/R.sub.H-sat !.times.100%. When the material is layered, the resistance change is further proportional to the cosine of the angle between the magnetization directions of the alternate layers. Generally, the GMR effect has been observed to decrease as the temperature of the sample at the time of measurement increased. McGraw-Hill Yearbook of Science& Technology, 1995 (McGraw-Hill: New York)!.
For antiferromagnetically coupled multilayers, the sensitivity to magnetic fields is two orders of magnitude smaller than that of currently used anisotropic magnetoresistance (AMR) thin-film sensors. The resistance change of AMR sensors is 2% at room temperature in a 5 Oe field while the GMR antiferromagnetically coupled multilayers require a magnetic field of about 1 or 2 T to change resistance.
In 1992 two groups of researchers observed GMR in granular, inhomogeneous alloys of copper/cobalt (Giant Magnetoresistance in Heterogeneous Cu--Co Alloys, A.E. Berkowitz, et al., Phys. Rev. Let. 68(25): 3745, Jun. 22, 1992; J. Q. Xiao, et al., Phys. Rev. Let. 68(25): 3749 Jun. 22, 1992). In granular GMR materials small fields were sufficient to align the ferromagnetic particles. The GMR effect was observed at magnetic field magnitudes of the order of 5,000 Oersted (Oe) instead of 10,000 kiloOersted (kOe). Observing that the GMR in multilayered structures comes from the reorientation of single domain magnetic layers, compounds were formed that contained single domain magnetic particles (such as cobalt) in a nonferromagnetic matrix. In an alloy having a significant volume fraction of single domain magnetic particles, the magnetic orientation of the particles is random at the coercive magnetic field; there are many particles that are statistically arranged antiparallel or less, so that a smaller applied magnetic field produces GMR. Thus it became possible to make materials having GMR properties at fields of 5 kOe and less. In a granular, inhomogeneous material, the magnitude of the GMR effect is controlled by the volume fraction of magnetic particles in the material, the magnetic particle size, the magnetic particle density, interface roughness, and impurities in the nonmagnetic metallic matrix.
GMR materials have in the past been constructed using sputtering, sputtering from a single composite target, evaporation, metal pastes, mechanically combining the magnetic and nonmagnetic materials, or implanting the magnetic materials (in the form of ions) into the nonmagnetic matrix. All of these methods are too costly in terms of equipment required, labor effort, and fabrication time, to be commercially useful.
It would be very desirable to be able to make a material that exhibited GMR properties at room temperature in a magnetic field of 5 kOe or less for a reasonable cost.